Such tensioning devices are wide-spread and are in particular employed as chain tensioners in timing chain drives of internal combustion engines. Usually, they are arranged in the loose side of the chain drive, as during normal operation lower tension forces occur there on average, so that the pressure spring only has to be designed for these lower tension forces. These tensioning devices normally comprise a housing with a piston bore in which a tensioning piston is arranged which is pretensioned by means of the pressure spring. The hollow cylindrical tensioning piston forms a pressure chamber together with the piston bore of the housing, which chamber is filled with a hydraulic fluid for dampening the retraction motion of the tensioning piston. The pressure chamber is in communication with a hydraulic cycle, e.g. the engine oil cycle of an internal combustion engine, via a check valve to replace the hydraulic fluid escaping from the pressure chamber via a leakage gap formed by the piston and the tensioner housing, or via a corresponding vent hole.
Such tensioning devices often comprise locking means which prevent the retraction of the tensioning piston into the housing beyond a predetermined stop but permit further readjustment of the tensioner, e.g. in case of chain wear. For this, a front region of the tensioning piston which is not exposed to the pressure chamber is conventionally provided with a lateral toothing cooperating with a ratchet element. The pretensioned ratchet element permits the tensioning piston to slip through in the tensioning direction of the pressure spring and simultaneously prevents excessive retraction of the tensioning piston into the piston bore of the housing in case of relatively hard and strong vibration impacts. Such a tensioning device is known from DE 10014700 A1, where the piston pretensioned by the spring comprises two ratchet sections essentially arranged oppositely and in which the serrated profiling of the separate ratchet element engages.
Moreover, there are also tensioning devices with locking means which take care that a latching effect occurs and the tensioning piston can no longer be retracted in the piston bore when the pressure in the hydraulic cycle is not high enough, e.g. when the engine is being switched off and started. Such a tensioning device is known from EP 657662 A2. This tensioning device comprises a tensioning piston having several catch grooves at its outside circumference into which a spring-loaded catch piston engages. The front face of the catch piston is beveled and the pressure of the hydraulic medium can act on it so that it becomes disengaged. By the locking position of the catch piston, a certain degree of pretension is maintained even while the engine is standing still, despite a possible leakage of the hydraulic medium from the pressure chamber. With each new start up of the tensioning device, a certain tension therefore exists independently of the pressure in the hydraulic cycle. As soon as in the hydraulic cycle and thus also in the pressure chamber of the tensioning device sufficient pressure has built up, the hydraulic pressure also disengages the latching means and the tensioning piston operates in a conventional manner.
A generic tensioning device is known from DE 3636918 A1. The tensioning piston that can be extended from the piston bore of a fixed adjuster housing acts on the flexible drive means in the tensioning direction. At the first startup of the tensioning device, the locking ring arranged on the piston snaps into a first catch position of the adjuster housing. If the pressure of the hydraulic medium in the pressure chamber is no longer sufficient to stop the piston during a retraction motion, e.g. during startup or in case of strong vibration impacts, the locking ring hits a stop edge of the piston, whereby the retraction motion of the piston into the piston bore is limited. If in case of wear of the drive means the initial position of the locking ring is no longer sufficient to define a suited working range of the tensioning piston, the locking ring is automatically moved to the next catch point of the adjuster housing by a second stop edge.
Such tensioning devices with ratchet elements or locking rings to limit the retraction motion of the tensioning piston into the housing are wide-spread and have proved themselves in use. The corresponding constructions of the locking means have a relatively elaborate design, where a partially complex manufacture and the observation of narrow tolerances are required for secure function.
Furthermore, these locking means usually consist of a series of components that have to be assembled in a complex manner and locked in a transport and/or assembly position.